This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying regions of rock formations from which hydrocarbons may be produced.
Hydrocarbon saturation S.sub.O is generally determined from a measured water saturation S.sub.W as follows: EQU S.sub.O =1-S.sub.W . ( 1)
Water saturation present in a subterranean formation is typically determined from interpretation of conventional electrical (i.e., resistivity) logs recorded in a borehole drilled through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation se forth in "The Electrical Resistivity Log As An Aid In Determining Some Reservoir Characteristics", Trans. AIME, Vol. 46, pp. 54-62, 1942, by G. E. Archie. This equation is expressed as EQU S.sub.w.sup.n =R.sub.w /.phi..sup.M R.sub.t , ( 2)
where S.sub.w is the fractional water saturation (i.e. free and bound water of the formation expressed as a percent of the available pore space of the formation), R.sub.w is the formation water resistivity, .phi. is the formation porosity, R.sub.t is the formation electrical resistivity, n is the saturation exponent and m is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict fractional water saturation S.sub.w from the formation resistivity, R.sub.t, using the equation in any of its forms.
Certain logs have provided formation resistivity R.sub.t and porosity .phi.. Water samples provide the best values for R.sub.w. Standard practice is to measure rock sample resistivities R.sub.o and R.sub.t for a number of water saturations and to plot the logarithm of I versus the logarithm of S.sub.w. Archie's equations assume such a logarithmic plot can be fit by a straight line with slope of -n.
When the physical properties of a core sample from a subterranean formation are isotropic, the formation resistivity R.sub.t measurement gives the same value regardless of how the core sample is oriented with respect to the larger rock sample from which such sample is obtained. Many core samples are, however, not homogenous and electrically isotropic and resistivity (or its reciprocal, conductivity) is not isotropic, but has different values according to the direction in which the core sample was taken. A conductivity computed from the sampled voltage and injected current multiplied by a geometrical factor gives a conductivity value which is a mixture of the conductivities in three orthogonal directions in the core sample. However, a commonly encountered form of an anisotropic medium in an earth formation is termed transversely isotropic having only two distinct values of conductivity. When sedimentary layers are visible, it can be assumed that conductivity in any direction parallel to the layering has a uniform value, while conductivity perpendicular to the layering has a different value. Conductivities measured in any other directions will exhibit a mixture of these two values. For such anisotropic core samples, the Archie saturation exponent n is strongly dependent on the direction the conductivity measurement is made and when such measurement is taken across permeability barriers within the core sample the saturation exponent may be one and a half times as large as if the measurements were taken parallel to the permeability barriers. This difference can have a large detrimental effect on the determination of hydrocarbon reserves derived from the calculated water saturation of equation (2). Previous methods for carrying out such measurements have required the use of a pair of cylindrical core samples or a single cubic core sample. Firstly, for the pair of cylindrical core samples, one is cut parallel to the bedding plane and the other is cut perpendicular to the bedding plane. Two difficulties are inherent; one is that the pair of core samples may not be identical in all respects except for the direction of the planes relative to the cylindrical axes of the core samples, and the other is that it would be extremely difficult to obtain the same partial water saturations in each core sample for comparison measurements. Secondly, for the single cubic core sample, the sample is cut with the bedding plane parallel to two faces of the cube and normal to two faces of the cube. This makes it difficult to carry out conductivity measurements at in-situ pressure.
It is, therefore, an object of the present invention to determine the tensor components of conductivity of a single cylindrical core sample that is electrically anisotropic and to identify the degree of anisotropy changes as the brine saturation of the core sample changes so that an accurate water saturation can be calculated from equation (2).